1. Field of the Invention
This invention relates to a finder optical system, and more specifically to a finder optical system for a single-lens reflex camera.
2. Description of the Prior Art
The picture quality obtainable with silver-salt photosensitive materials has in recent years been improved to such an extent that it is now practicable to employ disk films with a format as small as 8 by 11 mm. Moreover, there have been marketed and announced video and electronic cameras employing image pickup tubes and solid-state image pickup devices rated as small as 2/3" and 1/2". As the picture size of these cameras is smaller than the 24 by 36 mm size of the 35 mm camera, it is necessary for the purpose of obtaining high image magnification to make the focal length of the eyepiece as short as possible so as to increase its magnification as a loupe. Particularly in the case of a camera using an image pickup tube or solid-state image pickup device, however, it is not possible to locate the eyepiece immediately behind the prism since a considerable amount of space is inevitably taken up by electronic circuitry, the drive section, the image recording section and the like. Therefore, when an eyepiece of conventional construction like that shown in FIG. 1 is used, it can be located in the desired position only if its focal length is made long and it therefore becomes impossible to avoid a reduction in image magnification. For example, taking D/f (where D is the diagonal length of the picture plane and f is the focal length of the eyepiece) as an index of the image magnification, in the ordinary single-lens reflex camera where D=43.2 and f=58, it is possible to obtain a value of D/f of approximately 0.74, whereas when the same construction is used in a 2/3" video camera, the result becomes D/f=11/58.apprxeq.0.19. In the latter case, therefore, the image magnification becomes very small and is totally unsatisfactory. For practical purposes it is necessary to realize a value of D/f of not less than around 0.4.